This invention relates generally to ultrasonic flow-meters for measuring flow rate, and in particular to a Doppler-type meter which is corrected for errors arising from changes in the flow velocity profile of the fluid being metered to provide more accurate readings than standard meters of the same type.
In general, ultrasonic flowmeters are either of the through-beam or of the Doppler-type. In a through-beam meter, the meter pipe has upstream and downstream transducers mounted thereon which are alternately excited to generate an ultrasonic pulse which is propagated through the fluid being metered and received by the other transducer. The upstream propagation time minus the downstream propagation time represents the time delay difference between the generation of the emitted pulses and their reception. This difference is a function of the flow velocity of the fluid and is convertible into a flow rate reading.
The through-beam ultrasonic flowmeter is acceptably accurate only when the liquid being metered is substantially free of contaminants; for the presence of particulate matter in the propagation path causes this meter to malfunction. In contradistinction, contaminants in the fluid are essential to the operation of a Doppler-type meter.
The Doppler effect is encountered whenever a wave source generating sonic, radio-frequency, light or any other form of wave activity and a wave receiver are in relative motion with respect to one another. When the distance between the source and its receiver is decreasing, extra waves are detected in a given time, resulting in an apparent increase in the received frequency. When, however, the distance is increasing, an apparent decrease is experienced in the frequency of the received signal.
Thus in a continuous wave Doppler-type radar system, the projected radar wave is reflected by a moving target and picked up by a radar receiver, so that the distance travelled by the radar wave between the transmitter and receiver varies as a function of target movement, the resultant difference between the transmitted and received frequencies being proportional thereto. Similarly, in a Doppler-type ultrasonic flowmeter, a shift in received frequency results from the reflection of a projected beam of ultrasonic energy by bubbles, particles or other contaminants carried by the fluid being metered.
In a Doppler-type ultrasonic flowmeter, the transmitted signal is mixed with the received signal to produce a beat signal which represents the difference therebetween, the frequency of the beat signal being proportional to the velocity of the reflecting contaminants and hence to the flow rate of the fluid through the flow pipe.
Because in a Doppler-type ultrasonic flowmeter the transmitting and receiving transducers are mounted on the exterior of the meter pipe, the instrument is obstructionless and is free of erosion and corrosion problems. The normal fields of application for a Doppler-type flowmeter are pipes conducting contaminated media where through-beam flowmeters do not work or perform poorly. Thus Doppler-type ultrasonic flowmeters are useful in metering raw sewage or sludge in water treatment plants as well as in metering wash or wastewater, slurries and effluents in chemical, paper processing and mining systems.
As noted in the article by Morris in the August 1979 issue of Control Engineering, entitled "What's Available in Ultrasonic Flowmeters,"0 the accuracy of Doppler-type flowmeters is normally less than that quoted for through-beam meters. Thus, among the through-beam strap-on type commercial meters identified by Morris is an instrument produced by Controlotron Corporation, whose specifications indicate an accuracy of .+-.0.5 to .+-.1.5%. Also identified by Morris among the strap-on Doppler-types is a Leeds & Northrup instrument whose specifications indicate an accuracy of .+-.2.6 to .+-.5%.
The reason for the relative inaccuracy of the Doppler-type ultrasonic flowmeter has not heretofore been understood. The Morris article attributes this deficiency to the inherent inability of the meter to determine the location of the contaminants in the fluid system, but this is not the crux of the problem.
In a Doppler-type flowmeter, reflection from the contaminants in the fluid conducted by the pipe occurs in a zone in which the diverging beam projected by the transmitting transducer intersects the reception beam converging toward the receiving transducer. If within this zone of intersection, the contaminants carried by the fluid were all travelling at the same velocity, then the resultant negative beat between the transmitted and received signal would be a single frequency accurately indicative of flow rate of the fluid. But, in reality, the contaminants in the stream do not usually move at the same velocity, which is why in existing Doppler-type flowmeters, the readout is somewhat misleading.
The velocity of the contaminants depends on the flow profile of the stream, which in turn depends on whether the flow is laminar or turbulent, and on the Reynolds number, so that in many instances the contaminants within the zone of intersection in which reflections are obtained are moving at disparate velocities to produce not a single beat frequency, but a band of beat frequencies. Hence a Doppler-type ultrasonic flowmeter which operates on the assumption that only a single beat frequency is produced, will inevitably be inaccurate.
Velocity profile is the representation of the fluid velocity distribution in a plane perpendicular to the flow direction. Thus an axisymmetrical velocity profile is one in which the velocity is constant at any given radius measured in a direction perpendicular to the flow axis from the center of the flow pipe. Velocity profile is a function of the viscosity of the fluid as well as of the shape of the meter pipe and the presence of disturbances in the pipe line, such as protrusions, elbows, reducers or other discontinuities.
A Doppler-type ultrasonic flowmeter which fails to take velocity profile into account will inherently be inaccurate; for the contaminants in the flow stream do not all move at the same velocity, and unless the velocity profile is determined from which the mean velocity can be calculated, an acceptably accurate reading is not possible.